Fusion is a commonly performed procedure for bonding adjacent bone structures of the spine and for those not adjacent, providing long term replacement by fixation and support to treat degenerative or deteriorated disorders of the spine.
An inter-vertebral disc is a ligamentous cushion disposed between vertebrae and as a result of injury, disease or other disorders may deteriorate in part or in whole, thereby leading to mechanical instability and painful disc translocations and often necessitating bed rest or hospitalization. If left untreated possible subsequent involvement of adjacent nerves and muscular involvement may occur. In such cases, if treatment is delayed, permanent damage to the nerves may result in muscular atrophy and severe dysfunction.
Procedures for disc surgery may involve partial or total excision of the injured disc portion and replacement with biocompatible devices of bone, elastomeric or other bio-compatible materials.
Corpectomy is a procedure performed to relieve pain caused by fractured vertebrae and subsequent decompression of the spinal cord when the entire anterior body of the vertebrae is involved. This is most common in vertebral fractures referred to as Burst fractures and usually occurs from severe trauma, such as a motor vehicle accident or fall from a height resulting in a great deal of force directed vertically onto the spine causing a vertebral body to be crushed in all directions. A vertebral body that is crushed in such instances requires immediate surgical intervention once the patient has been stabilized so as to prevent irreversible damage to the spinal cord. Other fractures of the vertebral body may occur only in the anterior portion and are known as compression fractures, and as such do not require decompression of the spinal cord. In such instances, only a portion of the vertebral body and the entire adjacent vertebral discs are removed and an implant may be placed between the adjacent vertebrae for fusion to occur in restoring the required support and curvature of the spine. Corpectomy may also be performed in individuals having bone spurs that put pressure on the nerves or spinal cord. Other indications for a corpectomy surgical procedure are diseases due to infection or malignant tumors resulting in degeneration of the vertebral bone body. Corpectomy may also be the surgical method of choice in treatment of kyphosis or lordosis in the cervical, thoracic and lumbar areas so as to restore the normal curvature of the spine.
Previous procedures for corpectomy involved total or partial excision of the diseased or injured vertebral body along with the adjacent vertebral discs and replacement with biocompatible devices of bone, or bone-like material or artificial biocompatible implants that may be adjustable or non-adjustable for fusion to the adjacent vertebrae.
Back in 1983, Dubousset, with the assistance of Graf and Hecquet, defined the three-dimensional relationship of the spine in space and states: “after fusion of the spine, the resulting balance is determined by the unfused segments of the spine remaining mobile, not by the fused segments” and continues by stating “the dynamic changes above and below proposed end vertebrae of a spinal fusion are more important in determining final balance than what occurs within the fused segments.” A publication entitled “Journal of Neurosurgery; Spine,” September 2004 Volume 1 Number 2, in further support of Dubousset's Three-Dimensional Theory states: “[t]he normal motion of a lumbar segment includes independent translation and rotation in all three planes of motion (flexion-extension, lateral bending, and axial rotation). Normal motion is often represented as a factor of coupled motion in two planes. The implant-related geometrical configuration and material would determine the static configuration, dynamic motion, schematics, and any constrained nature of the motion. The exact placement of the artificial lumbar disc in the disc space is determined by its biomechanical design.” Although these references are directed to artificial disc biomechanics, they are also directly related to corpectomy which normally involves the removal of the diseased vertebral body and its two adjacent vertebral discs. Corpectomy results in a significantly large space between the remaining intact adjacent vertebral bodies and fusion of the space results in extremely heavy loads and shearing forces directed at the fused sites and at the implants used for fusion at said sites. An article entitled “Prediction of Mechanical Behaviors at Interfaces between Bone and Two Interbody Cages of Lumbar Spine Segments,” identifies that micromotion at bone-implant interfaces can hinder bone growth into the surface pores of an implant and that relative micromotion is sensitive to the friction coefficient of the interfaces, the bone density, and the loading conditions. An article published in Spine 2000 Dec. 1, entitled “2000 Volvo Award Winner in Biomechanical Studies Monitoring in Vivo Implant Loads with a Telemeterized Internal Spinal Fixation Device” identifies that implant loads often increased shortly after anterior interbody fusion was performed and that a flexion bending moment acts on an implant even with the body in the relaxed lying position.
Early Techniques
Bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebrae and the spinal column was stabilized by way of a plate or rod connecting the involved vertebrae. However, the use of bone may require undesired additional surgery and is of limited availability in its most useful form. In addition, the risk of infection and rejection is a significant consequence. In any event, bone is only marginally structural and with bone it is difficult to stabilize both the implant against dislodgment and stabilize the adjacent vertebrae. It becomes desirable to find solutions for stabilization of an excised disc space by fusing an artificial disc to the vertebrae between their respective end plates so that said vertebra can function in a relatively normal fashion by movement subsequent to load forces placed upon the spine. Following corpectomy, it is also desirable to find solutions for stabilization of the space resulting from the excised discs and diseased vertebral body by an implant system that fuses one or more artificial discs of the implant to the vertebrae between their respective end plates of the artificial discs so that the vertebrae can function in a relatively normal fashion by movement subsequent to load forces placed upon the spine. And more particularly to the rigid adjustable vertebral intermediate body to which the artificial discs are attached.